Very fast optical flaring from a possible new Galactic magnetar

Rapid optical flaring of an unprecedented type was detected from a transient Galactic high-energy source, SWIFT J195509.6+261406[1]. On June 10, 2007, Swift-BAT triggered on GRB 070610, which turned out to be a previously unknown X-ray transient in the Galaxy. Optical emission following this transient was observed after only 421 s with the high-time-resolution single-photon counting photometer OPTIMA. Measurements continued for the following 5 nights.We detected very strong optical flares (>6 mag) with extremely short timescales: duration of individual flares 2-100 s, shortest variability timescales 0.4 s. The scale and magnitude of the observed variability combined with a distance estimate of 4-8 kpc indicate a non-thermal origin of the observed radiation. The morphology of the optical flares is reminiscent of X-ray outbursts of SGRs. The time resolution and high signal-to-noise ratio during the brightest optical outbursts allow to compute their Fourier power spectral density. Features similar to QPOs appear at periods of 6-8 seconds, typical rotational periods for magnetars. X-ray observations independent from our optical analysis show hints of periodicity at a coinciding frequency. We conclude that the timing properties of the fast, bright outbursts of SWIFT J1955 suggest a connection between this transient and magnetars flaring in the optical.